1. Field of the Invention
The present invention is directed to the field of exhaust gas chemistry responsive sensors. More particularly, the present invention is directed to that portion of the above-noted field which is concerned with the construction of an exhaust chemistry responsive sensor for insertion in the exhaust system of an automotive internal combustion engine to provide a signal indicative of the air to fuel ratio of the combustion mixture providing the exhaust gases. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of an exhaust gas sensor which may be used to indicate a condition of stoichiometry in the combustion mixture which is generating the exhaust gases as a by-product of combustion and which sensor may be used as an input device for an air/fuel ratio controller such that the combustion mixture may be maintained at stoichiometry. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with providing an exhaust gas chemistry responsive sensor which will be substantially lower in cost and of substantially less complexity than prior exhaust gas sensors.
2. Description of the Prior Art
There are, generally speaking, two classes of exhaust gas sensors. Each makes use of a material which respond principally to the partial pressure of oxygen in the exhaust gases. The first of these, which is exemplified by the use of zirconia as the operative material, responds to a differential partial pressure of oxygen between a reference source of gas such as the atmosphere and a sensed gas to generate a galvanic voltage or electromotive force between the surfaces of the material which are exposed to the two gases, which may be used as a signal. These devices require that the surfaces exposed to the gases be provided with porous electrodes and that one surface be exposed to a relatively constant reference source while the second surface is exposed to the exhaust gases. This requirement presents constructional problems since it is normally the practice to use ambient air as the reference gas and this introduces substantial temperature gradients across the ceramic material. In order to provide rapid response times and for various other desirable operating characteristics, the zirconia material is preferably kept thin. The above-noted requirement and the preferred thinness also present sealing problems as well as other problems of an electrical nature. As a result, this class of devices tends to be fragile, expensive and relatively unreliable after being in use for a term of time less than that required to give an average of about 50,000 driving miles of service.
A second group of exhaust gas chemistry responsive sensors, which may be typified by the use of for example titania ceramic material as the operative material, exhibits an electrical resistance which varies, at elevated temperatures, as a function of the partial pressure of oxygen in the gaseous environment of the ceramic and as a function of temperature. The above-noted copending commonly assigned patent applications describe various titania ceramic exhaust gas sensor configurations which typically utilize an electrical heat source to provide the sensor with an initial heating and to thereafter maintain the sensor at a specific selected elevated temperature so that resistance variations will not be caused by fluctuations in the exhaust temperature.
Electrical heating means are typically provided in the form of an electrical resistance coil formed of platinum conductive wire. Such a heat source contributes substantially to the cost of a sensor, both from the standpoint of the cost of the platinum material and from the standpoint of the manufacturing complexity presented by the necessity of mounting the heater and communicating the heater, through the support ceramic material, to a separate electrical source for energization. Precise temperature control is required to eliminate temperature variations from influencing the sensor signal and to provide a very accurate temperature control particularly for operation of the associated internal combustion engine at nonstoichiometric combustion mixture ratios.
Investigation of the electrical resistance versus air/fuel ratio response curve of titania exhaust gas sensors has indicated that the resistance value of the titania varies substantially for the exhaust by-products of combustion mixtures which experience a lean to rich or rich to lean excursion or transition. In many instances, this variation may be several orders of magnitude, even in the face of adverse temperature variations. It is therefore an object of the present invention to provide a titania-based exhaust gas partial pressure of oxygen sensor to operate in the exhaust system of an engine operated with a combustion mixture having a stoichiometric air/fuel ratio which is low in cost and relatively simple to assemble.
One continuing objective which the automotive industry in general has in fabricating any power train related component is maximum durability. The federal law has further stimulated the automotive industry to attempt to obtain, in the case of pollution control related engine components such as an exhaust gas sensor for use in a feedback air/fuel ratio control system, a durability factor which would be equivalent to operation of the average vehicle over approximately 50,000 miles. Under such a requirement, an exhaust gas sensor would be required to undergo a large number of thermal cycles and considerable vibration as well as being required to withstand the extremes of seasonal weather contaminates to which a vehicle may be subjected. Such a device, in order to be cost effective, would have to achieve the desired level of operation and reliability while maintaining as low a cost as possible. Since the sensor and its associated mechanical hardware would be subjected to the high temperature environment of the exhaust system and could be expected to be subjected to exposure to road salt and the like it would be necessary that the electrical portion of the sensor be capable of withstanding thermal cycling in the presence of a salt environment. Conventional means of thermal and environmental insulation would not normally be expected to hold up to this type of environment and the number of electrical leads associated with the exhaust gas sensor would multiply the statistical chances of failure. It is therefore a further and specific object of the present invention to provide an exhaust gas chemistry responsive sensor requiring only a pair of electrical leads which may be arranged in such a fashion as to assure maximum protection against salt, road spray and splash. While these objectives can be achieved with the sensor construction according to the prior art by merely adding substantial insulation, this approach would greatly increase the cost and complexity of the devices. It is therefore a further and specific objective of the present invention to provide a low cost, low complexity exhaust gas sensor of the variable resistance type to operate as a stoichiometry indicator in an exhaust gas feedback responsive air/fuel ratio controller. More particularly still, it is an object of the present invention to provide an exhaust gas sensor of rugged construction which is low in cost and which is of sufficiently simple construction that it may be manufactured on largely automated machinery.
Since the partial pressure of oxygen responsive sensor must be connected to further electrical apparatus in order to provide input information, for example for the air/fuel ratio controller, the sensor must be provided with an electrical terminal which is plug compatible with this further apparatus. The leads normally embedded within the variable resistance sensor material are typically platinum. It is relatively very expensive to extend the platinum material all the way from the sensor wafer to the terminal. It therefore is necessary to provide an interconnection between the sensor wafer leads and the terminal leads as close to the wafer as possible in order to minimize the quantity of platinum conductor required for each sensor device. This interconnection must provide for ideal electrical contact between the sensor leads and the terminal leads, must be readily accomplished during manufacture of the sensor, and must be capable of withstanding the high temperature environment of the sensor device and the thermal cycling of the sensor device. The normal approach to designing an exhaust gas sensor for reduced cost and potential automated assembly would be to minimize the number of components and to place all connections between electrical conductors at the rear of the device to reduce temperature effects. However, exhaust gas sensors fabricated according to this philosophy have exhibited electrical failures resulting from shorts occurring at the electrical interconnections. These electrical shorts occurred in part as a consequence of the accumulation of material over a long period of time in the vicinity of the electrical interconnections. It is therefore a specific object of the present invention to provide a low cost, durable exhaust gas sensor construction which avoids the use of interconnections between electrical conductors exterior of the device. It is also an object of the present invention to provide an exhaust gas chemistry responsive sensor which intercommunicates a wafer of variable resistance exhaust gas chemistry responsive material with an electrical terminal in such a manner that cost and manufacturing complexity of the end device may be minimized while providing for an electrical interconnection between a precious metal conductor material and less expensive conductor material which may be accomplished within the body of the wafer support ceramic material without adversely altering the capability of the device to withstand extremes in temperature and thermal cycling.